The specific surface area of zirconia alone used as a catalyst carrier is only about 100 m2/g at 400° C. Zirconia alone having a specific surface area greater than 100 m2/g is generally amorphous that does not have a stable structure. Therefore, when zirconia alone is used as a catalyst carrier, its specific surface area reduces in size at a temperature higher than 400° C. This makes it difficult to obtain stable catalytic performance at a high temperature. In order to satisfactorily function as a catalyst carrier, a higher heat resistance (thermal stability) is required.
In contrast, a zirconia-ceria composition consisting of a zirconium oxide and a cerium oxide generally maintains a relatively large specific surface area even at a high temperature of 1,000° C. Because of this property, a zirconia-ceria composition is more advantageous than zirconia alone in terms of thermal stability, when used as a catalyst.
However, not only the thermal stability so as to maintain a certain specific surface area but also the pore volume distribution, thermal stability of the pores, etc., have recently become increasingly important. This is because when a carrier supports an active precious metal, the zirconia particles form agglomerations and the number of pores, in particular those having a diameter of 100 nm or more, is reduced when heat-treated, and the particles of platinum, rhodium, palladium and like precious metals supported on the surface of the carrier may be embedded in an inside of zirconia particles, decreasing their ability to effectively contribute to the reaction occurring on the surface.
Specifically, precious metals, which are active species of catalysts, can be supported with sufficient dispersibility in pores having a diameter of 10 to 100 nm. Therefore, it is preferable that the volume of pores having a diameter of 10 to 100 nm be as large as possible, and that of the volume of pores having a diameter of 100 nm or more be as small as possible. It is more preferable that the pores having a diameter of 10 to 100 nm have sufficient thermal stability against temperatures as high as 1,000° C. or more, so that the large volume of pores having a diameter of 10 to 100 nm can be maintained after the heat treatment.
Patent Document 1 discloses cerium oxide, zirconium oxide, (Ce,Zr)O2 compound oxide, and solid solution (Ce,Zr)O2 having about 0.8 ml/g or more pore volume after calcining at about 500° C. in air for 2 hours.
Patent Document 2 discloses mixed cerium and zirconium oxide having at least 0.6 cm3/g of total pore volume, wherein at least 50% of the total pores have a diameter of 10 to 100 nm. Examples of D2 disclose compound oxides having about 0.8 cm3/g of pore volume after baking at 800° C. for 6 hours.
However, taking into consideration that automobile catalysts are actually used under temperatures not less than 1,000° C., the compound oxides disclosed in Patent Documents 1 and 2 do not have satisfactory high temperature thermal stability.
Patent Document 3 discloses a zirconia-based porous material having a pore diameter peak at 20 to 110 nm in the pore distribution measured by the BJH method, wherein the total pore volume is 0.4 cc/g or more, and the total volume of pores having a diameter of 10 to 100 nm is 50% or more of total pore volume.
Patent Document 4 discloses porous zirconia-based powder having a total pore volume of at least 0.75 ml/g after a heat treatment at 1,000° C. for 3 hours, and the total volume of pores having a diameter of 10 to 100 nm after the heat treatment at 1,000° C. for 3 hours is at least 30% of total pore volume. Patent Document 4 also discloses a method for producing porous zirconia-based powder comprising the steps of “adding a sulfatizing agent to a zirconium salt solution to produce a basic zirconium sulfate; neutralizing the basic zirconium sulfate to produce a zirconium hydroxide; and heat-treating the zirconium hydroxide to produce a porous zirconia-based powder, wherein the sulfatizing agent is added to the zirconium salt solution having a temperature 100° C. or more in an autoclave”. However, in Patent Documents 3 and 4, since the volume of pores having a diameter of 100 nm or more is as large as 0.3 ml/g, further improvement is required.
Patent Document 1
Japanese Unexamined Patent Publication No. 2001-524918
Patent Document 2
Japanese Patent No. 3016865
Patent Document 3
Japanese Unexamined Patent Publication No. 2006-36576
Patent Document 4
Japanese Unexamined Patent Publication No. 2008-081392